Each programmer will, of course, have his or her own preferences in regards to formatting, but there are some general guidelines that will make your programs easier to read.
  1. Just because you CAN do something a particular way doesn't mean that you SHOULD do it that way. Perl is designed to give you several ways to do anything, so consider picking the most readable one. For instance open(FOO,$foo) || die "Can't open $foo: $!"; is better than die "Can't open $foo: $!" unless open(FOO,$foo); because the second way hides the main point of the statement in a modifier. On the other hand print "Starting analysis\n" if $verbose; is better than $verbose && print "Starting analysis\n"; since the main point isn't whether the user typed -v or not.

    Similarly, just because an operator lets you assume default arguments doesn't mean that you have to make use of the defaults. The defaults are there for lazy systems programmers writing one-shot programs. If you want your program to be readable, consider supplying the argument.

    Along the same lines, just because you can omit parentheses in many places doesn't mean that you ought to:

    	return print reverse sort num values array;
    	return print(reverse(sort num (values(%array))));
    When in doubt, parenthesize. At the very least it will let some poor schmuck bounce on the % key in vi.

    Even if you aren't in doubt, consider the mental welfare of the person who has to maintain the code after you, and who will probably put parens in the wrong place.

  2. Don't go through silly contortions to exit a loop at the top or the bottom, when perl provides the "last" operator so you can exit in the middle. Just outdent it a little to make it more visible:
    	for (;;) {
    	last line if $foo;
    	    next line if /^#/;
  3. Don't be afraid to use loop labels--they're there to enhance readability as well as to allow multi-level loop breaks. See last example.
  4. For portability, when using features that may not be implemented on every machine, test the construct in an eval to see if it fails. If you know what version or patchlevel a particular feature was implemented, you can test $] to see if it will be there.
  5. Choose mnemonic identifiers.
  6. Be consistent.